The present invention relates to polymeric compositions and their uses, and more particularly to predominantly polypropylene-based, crosslinked compositions and articles suitable for use in a range of applications, including coating or insulating materials.
Polypropylenes are ideally suited to the preparation of coatings and insulations designed for use at operating temperatures in excess of those that can be withstood by other polyolefins, for example, polyethylene, which exhibit lower softening and melting temperatures. Polyethylenes have a maximum melting point, as measured by differential scanning calorimetry (DSC), of about 135xc2x0 C., whereas polypropylenes can have melting points as high as 175xc2x0 C.
Other attractive features of polypropylenes are their high rigidity and toughness, low cost and relatively low density. Applications for these coatings and insulations would include polymeric insulation for electrical wires and cables, and heat-shrinkable protective sleeves for high-temperature transmission pipelines, or applications requiring greater toughness and rigidity than is afforded by polyethylene-based systems. For example, heat-shrinkable sleeves used for the corrosion protection of high temperature pipeline joints are required to maintain dimensional stability and integrity at the operating temperature of the pipeline. Hence it is necessary to use a material, such as polypropylene, with a softening temperature or melting point high enough to prevent creeping or sagging of the sleeve from the pipe at the continuous operating temperature of the pipeline.
In order to maximise heat-resistance and physical properties, it is necessary to impart some thermoset characteristic to the material. This is done by crosslinking the polymer to some required degree. Crosslinking is also necessary for the production of heat-shrinkable articles to impart controlled shrinkage characteristics.
Polymers in which the predominant chain units comprise an alpha olefin, such as polypropylenes, are known to preferentially depolymerise when exposed to free radicals required to effect crosslinking. Hence, unlike polyolefins such as polyethylenes and copolymers of polyethylene, it is not possible to crosslink polypropylene-based materials to satisfactory levels, as is required, for example, in the production of wire and cable insulations, and heat-shrinkable articles, such as tubing, sheet, and moulded shapes, by using standard free-radical methods of crosslinking, such as electron beam irradiation or peroxide initiated crosslinking.
Alternate methods for crosslinking polyolefins are known. One such method comprises grafting a vinyl silane onto an olefin homopolymer or copolymer, followed by catalyzed hydrolysis and condensation of the silane groups. This is described in U.S. Pat. No. 3,646,155 (Scott), issued on Feb. 29, 1972. Alternatively, the vinyl silane may be copolymerised directly with olefin monomers as described in U.S. Pat. No. 4,413,066 (Isaka et al.), issued on Nov. 1, 1983. Since these methods require a free-radical generator to initiate the silane grafting or copolymerisation reaction, polypropylene is also unsuited to these methods of crosslinking.
Although the prior art contains examples of polypropylene-containing compositions having some crosslinking, none of these compositions is a predominantly polypropylene-based, crosslinked composition having the desirable properties of polypropylene.
One example of a prior art composition is disclosed in International Publication No. WO98/23687 (Laughner et al.), published on Jun. 4, 1998. This publication relates to the production of thermoplastic elastomers (TPE""s), which comprise a thermoplastic matrix substantially free of crosslinking and a discontinuous elastomer phase in the form of domains dispersed throughout the matrix. According to Laughner et al., the thermoplastic phase may comprise polypropylene and the elastomer domains comprising the discontinuous phase are grafted with silane and subsequently crosslinked to improve impact properties of the TPE. However, even after crosslinking, the continuous phase remains thermoplastic, thereby allowing the TPE to be re-melted for further processing. The materials taught by Laughner et al. do not have thermoset properties necessary for applications requiring high temperature resistance, and would be expected to creep, sag or melt upon exposure to temperatures experienced during operation of high temperature pipelines. Nor do the materials taught by Laughner et al. have thermoset properties required of heat-shrinkable articles, which must be heated above their melting point to effect shape recovery.
Another prior art composition is disclosed in U.S. Pat. No. 4,464,425 (Voigt et al.), issued on Aug. 7, 1994. This patent relates to foamed, rubber-like compositions produced from silane-grafted, crosslinked polymers. These compositions are disclosed as being useful in the manufacture of hoses, and would be expected to be flexible and rubber-like. These materials would not have the physical properties of a crosslinked, polypropylene-based composition.
U.S. Pat. No. 5,929,129 (Feichtinger), issued on Jul. 27, 1999, describes a process of blending a silane-grafted polyolefin with a non silane-grafted polypropylene. However, this patent is limited to the manufacture of foamed materials which do not have the physical properties of a crosslinked, polypropylene-based composition due to insufficient levels of crosslinking and blend mixture-compatibility.
The present invention overcomes the above-discussed problems of the prior art by providing predominantly polypropylene-based, moisture-crosslinked compositions and articles formed therefrom. The compositions according to the invention are comprised of a silane-modified ethylene-based polyolefin, at least 50 percent by weight of a non silane-modified polypropylene, and a silanol condensation catalyst.
Crosslinking provides the compositions of the invention with thermoset properties. The required degree of crosslinking is such that, when the composition is heated to a temperature above the crystalline melting point of the non silane-modified polypropylene, it is softened but does not become liquid.
The compositions according to the invention are suited for a wide range of applications, but are particularly well suited as coating or insulating materials, and are particularly well suited for use as sleeves for high temperature pipelines. The relatively high polypropylene content of the composition provides articles formed therefrom with a high softening or melting point, and the crosslinking provides sufficient thermoset properties to help stabilize the physical structure of the formed article at high temperatures, thereby preventing melting, sagging and creeping, and to permit the formed article to exhibit heat-shrinkability when heated above the crystalline melting point of the non silane-modified polypropylene. In other words, the compositions and articles according to the invention behave like crosslinked polypropylene compositions, which have not previously been produced due to the difficulties discussed above.
The composition according to the invention is preferably formed into a desired article by melt processing techniques such as extrusion and moulding, including multi-layer processing, for example co-extrusion of the blend with another material to form discrete but intimately bonded layers. The article thus formed is cross-linked in the presence of a silanol condensation catalyst under suitable conditions of heat and moisture, the catalyst being either blended with the composition during melt processing or added subsequently by coating the formed article, for example.
Accordingly, in one aspect, the present invention provides a crosslinked, predominantly polypropylene-based article comprising: a silane-modified ethylene-based polyolefin comprising either a silane-grafted polyolefin or a silane-olefin copolymer; at least 50 percent by weight of a non silane-modified polypropylene; and a silanol condensation catalyst; wherein the article is moisture crosslinked to a sufficient degree that it possesses thermoset properties such that, when the article is heated to a temperature above the crystalline melting point of the polypropylene, it is softened but does not become liquid.
In another aspect, the present invention provides a method for producing a crosslinked, predominantly polypropylene-based article comprising a crosslinked composition containing a silane-modified polyolefin and a non silane-modified polypropylene, said method comprising: (a) preparing said silane-modified polyolefin by reacting an ethylene-based polyolefin with a silane, said silane-modified polyolefin comprising either a silane-grafted polyolefin or a silane-olefin copolymer; (b) preparing a mixture comprising said silane-modified polyolefin, said polypropylene and a silanol condensation catalyst, said polypropylene being added in a sufficient amount that the article comprises at least 50 percent by weight polypropylene; (c) forming the article by melt processing the mixture prepared in step (b); and (d) crosslinking the article by exposing it to moisture at elevated temperature, wherein said crosslinking provides said article with thermoset properties such that, when the article is heated to a temperature above the crystalline melting point of the polypropylene, it is softened but does not become liquid.
As mentioned above, the composition according to the invention is comprised of a silane-modified ethylene-based polyolefin, at least 50 percent by weight of a non silane-modified polypropylene, and a silanol condensation catalyst.
Suitable non silane-modified polypropylenes for use in the composition of the invention are selected from the group comprising polypropylene homopolymers and copolymers of polypropylene with an olefin other than propylene. The non silane-modified polypropylene is preferably isotactic in nature, and preferably has a melt viscosity similar to that of the silane-grafted ethylene-based polyolefin, thereby ensuring maximum process compatibility.
The composition according to the invention is predominantly polypropylene-based, meaning that it comprises at least 50 percent by weight of the non silane-modified polypropylene, preferably with at least 50 percent of the monomer units in the composition being propylene monomer units.
More preferably, the composition comprises from about 50 percent to about 90 percent by weight, and even more preferably from about 60 to about 90 percent, of the non silane-modified polypropylene. Furthermore, where the non silane-modified polypropylene is a copolymer comprising propylene monomer units and units of some other monomer, the propylene monomer units preferably account for at least 50 percent of the total number of monomer units in the copolymer. More preferably, the propylene monomer units preferably account for at least 75 percent, and even more preferably 85 percent, of the total monomer.
In order to ensure that the composition according to the invention has a sufficiently high softening temperature, it is preferred that the melting point of the non silane-modified polypropylene, as measured by DSC, be at least about 145xc2x0 C. and more preferably at least about 150xc2x0 C., it being understood that the maximum melting point of polypropylene homopolymer is about 165xc2x0 C. It is also preferred that the non silane-modified polypropylene has a melting point, as measured by DSC, of at least about 20xc2x0 C. above the operating temperature of the article into which the composition is formed, which is usually in the range of from about 120xc2x0 C. to about 130xc2x0 C.
The silane-modified ethylene-based polyolefin comprises either a silane-grafted polyolefin or a silane-olefin copolymer. The process of chemically grafting silane molecules onto a polymer to form a silane-grafted resin, followed by catalysed hydrolysis and condensation of the silane, is a well known and established process, and is described in above-mentioned U.S. Pat. No. 3,646,155, which is incorporated herein by reference in its entirety. Preparation of silane-olefin copolymers is described in above-mentioned U.S. Pat. No. 4,413,066, which is also incorporated herein by reference in its entirety.
Suitable ethylene-based polyolefins for use in the composition of the invention are selected from the group comprising polyethylene homopolymers and copolymers of ethylene with an olefin other than ethylene having from 3 to 20 carbon atoms.
Preferred polyethylene homopolymers are selected from the group comprising low density polyethylene, high density polyethylene and linear low density polyethylene, with high-density polyethylene and linear low-density polyethylene being most preferred.
Preferred copolymers of ethylene are selected from those in which the olefin other than ethylene is selected from the group comprising propylene, butene, hexene, octene, ethylidene norbornene, vinyl acetate, methyl acrylate, ethyl acrylate and butyl acrylate. The copoylmer of ethylene may also comprise an ethylene-propylene or ethylene-propylene-diene elastomer. The copolymers of ethylene are preferably prepared using so-called metallocene catalysts. In a particularly preferred embodiment of the invention, the ethylene-based polyolefin comprises an ethylene-octene copolymer polymerised using a so-called metallocene catalyst.
The ethylene-based polyolefin preferably comprises from about 50 to about 100% by weight ethylene, more preferably from about 60 to about 90% by weight ethylene, and most preferably from about 80 to about 95% by weight ethylene. The density of the polyethylene or the ethylene co-polymer is preferably in the range of about 0.85 to about 0.95 g/cm3.
The silane-modified ethylene-based polyolefin is prepared by reacting the ethylene-based polyolefin in the molten state with an organic silane having the general formula RRxe2x80x2SiY2. R represents a monovalent olefinically unsaturated hydrocarbon or hydrocarbonoxy radical, Y represents a hydrolysable organic radical and Rxe2x80x2 represents a monovalent olefinically unsaturated hydrocarbon or hydrocarbonoxy radical, or a hydrolysable organic radical.
The monovalent olefinically unsaturated hydrocarbon or hydrocarbonoxy radical is preferably selected from the group comprising vinyl, allyl, butenyl, cyclohexenyl, cyclopentadienyl, or cyclohexadienyl radicals.
The hydrolysable organic radical is preferably selected from the group comprising: alkoxy radicals such as methoxy, ethoxy and butoxy radicals; acyloxy radicals such as formyloxy, acetoxy and propionoxy radicals; oximo radicals such as xe2x80x94ONxe2x95x90C(CH3)2, xe2x80x94ONxe2x95x90CCH3C2H5 and xe2x80x94ONxe2x95x90C(C6H5)2; and substituted amino radical selected from alkylamino and arylamino radicals such as xe2x80x94NHCH3, xe2x80x94NHC2H5 and xe2x80x94NH(C6H5)2.
More preferably, the organic silane has the general formula RSiY3, with the most preferred group R being the vinyl radical, and the most preferred Y group being the methoxy and ethoxy radical. Accordingly, the most preferred organic silanes for use in the present invention are vinyltriethoxysilane and vinyltrimethoxysilane.
The amount of silane reacted with the polyolefin depends in part upon the reaction conditions and the degree of modification desired in the polyolefin. The proportion may vary from about 0.1 to about 50% by weight based on the total weight of the silane-grafted resin, more preferably from about 0.5 to 10% by weight, and most preferably from about 1.0 to 2.5% by weight.
A free-radical initiator is also incorporated into the polyolefin to initiate the graft polymerization reaction. The free radical initiator is preferably an organic peroxide selected from the group comprising benzoyl peroxide, dichlorobenzoyl peroxide, dicumyl peroxide and di-tertiary butyl peroxide, with dicumyl peroxide being most preferred. The criteria for choosing an appropriate free-radical initiator are known to persons skilled in the art and are described in the above-mentioned U.S. Pat. No. 3,646,155 and will not be repeated here.
Preferably, the organic peroxide free-radical initiator is added in an amount of from about 0.1 to about 1.0% by weight of the silane-grafted resin, more preferably from about 0.05 to 0.2% by weight.
Silane-grafted polyolefins according to the invention are prepared by reacting the silane and peroxide with the ethylene-based polyolefin above the melting point of the polyolefin under conditions in which the polyolefin is subjected to mechanical working, using processes known to those skilled in the art. Silane-olefin copolymers according to the invention are prepared by reacting an unsaturated ethylene-based olefinic monomer with an unsaturated silane monomer containing one or more hydrolysable groups, preferably an organic silane as described above, such as vinyltrimethoxy silane.
The silane-modified ethylene-based polyolefin is subsequently melt-processed with the non silane-modified polypropylene, the silanol condensation catalyst, and one or more optional ingredients selected from the group comprising pigmenting agents, mineral fillers, flame-retardant additives, antioxidants, UV stabilisers, lubricants and compatibilisers.
The silanol condensation catalyst is preferably selected from the group comprising organic bases, carboxylic acids and organometallic compounds including organic titanates and complexes or carboxylates of lead, cobalt, iron, nickel, zinc and tin. More preferably, the catalyst is selected from the group comprising dibutyltin dilaurate, dibutyltin diacetate, dibutyltin octanoate, dioctyltin maleate, dibutyltin oxide and titanium compounds such as titanium-2-ethylhexoxide. The most preferred silanol condensation catalyst is dibutyltin dilaurate, though any material which will catalyse the silane condensation reaction is suitable for the invention. The condensation catalyst is preferably added in an amount of from about 0.01 to about 1 percent by weight of the coating material, more preferably about 0.05 to about 0.5 percent by weight, and most preferably about 0.1 to 0.2 percent by weight.
The optional compatibiliser may be selected from any of the ethylene-based polyolefins and polypropylenes described above. Preferred compatibilisers are selected from the group comprising ethylene-propylene copolymers; ethylene-propylene diene elastomers; polyolefins modified with reactive functional groups selected from the group comprising acrylic acids, methacrylic acids, acrylates, methacrylates and anhydrides; and block copolymers selected from the group comprising styrene-butadiene, styrene-butadiene-styrene, styrene-ethylene/propylene and styrene-ethylene/butylene-styrene.
Where a compatibilising agent is added to the composition of the invention, it is preferably added in an amount from about 1 to about 25 percent by weight, and such that the composition remains predominantly polypropylene-based.
Subjecting the composition produced by combining the ethylene-based polyolefin, the non silane-modified polypropylene and the silanol condensation catalyst to moisture, preferably at an elevated temperature, will induce crosslinking of the silane groups via a combined hydrolysis and condensation reaction. Atmospheric moisture is usually sufficient to permit the crosslinking to occur, but the rate of crosslinking may be increased by the use of an artificially moistened atmosphere, or by immersion in liquid water. Also, subjecting the composition to combined heat and moisture will accelerate the crosslinking reaction. Most preferably, crosslinking is effected at a temperature above 50xc2x0 C. and most preferably by exposing the composition to a temperature of 85xc2x0 C. and a relative humidity of 90% for approximately 100 hours.
A particularly preferred process for forming a crosslinked, predominantly polypropylene-based composition and article according to the invention will now be described below.
The ethylene-based polyolefin, silane, peroxide free-radical initiator and optional processing stabiliser are pre-blended and passed through an extruder at a temperature above the melting point of the polyolefin and above the decomposition temperature of the peroxide, in order to accomplish grafting of the silane onto the polyolefin. The grafted material is then passed through a multi-strand dye into a trough of cooled water, subsequently chopped into pellets with a strand pelletizer, and dried.
The grafted polymer pellets are then blended with the non silane-modified polypropylene, the silane condensation catalyst, an antioxidant stabiliser, as well as any of the optional ingredients described above. The amount of the non silane-modified polypropylene is sufficient to ensure that the final composition will be predominantly polypropylene-based. The mixture is melt processed and shaped to form an article, for example by extrusion, co-extrusion or moulding.
The shaped article is then exposed to moisture, preferably at an elevated temperature, to effect crosslinking of the composition. Shaped articles formed from the crosslinked, predominantly polypropylene-based composition, as well as the composition itself, exhibit the property of softening but not becoming liquid when re-heated above the crystalline melting point of the non silane-modified polypropylene. Thus, the composition according to the invention, as well as articles formed from the composition, exhibit thermoset properties.
The thermoset properties of the composition according to the invention make it desirable for the manufacture of heat-shrinkable articles, since the article, when softened by heating, may be stretched beyond its original extruded or moulded dimensions without rupture using relatively low forces, and can then be xe2x80x9cfrozenxe2x80x9d in its stretched state by cooling it rapidly to below the crystalline melting point. Stretching can be accomplished by mechanical, pneumatic or hydraulic means. After cooling, the stretched crosslinks are held in a stable state by the re-formed, solid crystalline regions. Subsequent re-heating of the stretched article above the melting point will cause the crystalline regions to re-melt and the structure to revert to its original extruded or moulded dimensions. The crosslinking also prevents the article from becoming liquid during this shrinking process.